1. Field of the Invention
The present invention relates to the field of sterilizing gases and more particularly to sterilization based on the use of ethylene oxide.
2. Description of the Prior Art
It is known in the art to sterilize articles by the application of boiling water or steam to the article to be sterilized. However, in the field of medicine among others, there is a need to employ sterilant compositions because many articles cannot withstand the temperature or moisture associated with steam sterilization. Sterilization with a germicidal agent, such as ethylene oxide gas or ethylene oxide containing gas mixtures has played an important role in sterilizing heat or moisture sensitive materials. Ethylene oxide is a widely used sterilant since it is both an inherently effective sterilant and its residues rapidly volatize from the article to be sterilized. Although ethylene oxide alone may be used to carry out sterilization, this is not done because ethylene oxide is a highly flammable gas. Ethylene oxide forms explosive mixtures in air from about 3.0 volume percent to 100 volume percent ethylene oxide. Thus, when ethylene oxide is used alone as a sterilizing gas, precautions such as explosion proof equipment are necessary. Therefore, ethylene oxide sterilant is generally used in a mixture with a flame retardant. Gaseous sterilization of reuseable medical and surgical equipment using a nonflammable mixture of ethylene oxide and a carrier gas has proven to be reliable, cost effective technology for many hospitals. The flame retardant component, must complement the properties of the ethylene oxide or the beneficial effects of the ethylene oxide will be lost. Inert carrier gases inhibit the flammability of ethylene oxide and provide sufficient autogeneous vapor pressure to deliver the liquid mixture from the source cylinder to the heat exchanger of the sterilizer vessel where the liquid mixture is vaporized. The most typical flame retardant chosen for use with ethylene oxide in a sterilant mixture has been dichlorodifluoromethane which known in the industry as CFC-12. The most commonly used sterilant mixture is a mixture of 12 weight percent ethylene oxide and 88 weight percent CFC-12. This mixture is commonly referred to in the industry as 12-88. In recent years CFC-12 has become undesirable since it is a chlorofluorocarbon which is believed to cause significant damage to the ozone layer in the upper atmosphere. Accordingly, worldwide reduction and elimination of the use of CFC-12 is now underway. This has created a problem for the use of ethylene oxide as a sterilant. Another flame retardant which is known for use with ethylene oxide is carbon dioxide. However a nonflammable ethylene oxide/carbon dioxide mixture contains less than 40 percent of the ethylene oxide per unit volume as does 12-88. Thus, sterilization must be carried out either at higher pressures or for longer contact times. Furthermore the large difference in the vapor pressures of ethylene oxide and carbon dioxide causes the mixture to separate as it is being withdrawn from the storage tank or cylinder, raising the danger of delivering a sterilant mixture rich in carbon dioxide, which will not sterilize, or rich in ethylene oxide, which is explosive.
As a result, improved sterilant mixtures employing ethylene oxide and other flame retardant halocarbons have been developed. These are exemplified in U.S. Pat. Nos. 5,342,579; 5,376,333 and 5,039,485 which are incorporated herein by reference. Although the major purpose of the inert carrier gas component in these sterilizing gas mixtures is to mask the flammability characteristics of ethylene oxide, simple substitution of an arbitrary nonflammable gas does not necessarily ensure a useful sterilizing gas mixture.
Almost universally, sterilant mixtures employing ethylene oxide and a flame retardant halocarbon are produced in manufacturing facilities whose piping and other vessels comprises carbon steel. In addition, the sterilant mixture is stored, transported and used in refillable pressurized cylinders comprising carbon steel. Unfortunately carbon steel eventually degrades to iron oxide. It has been found that this iron oxide, in both its alpha and gamma forms, assists in the degradation of ethylene oxide to reaction products of ethylene oxide such as acetaldehyde, ethylene glycol, polyethylene glycol and other reaction products. These reaction products form oils, contaminate the sterilant mixture, stain medical devices, clog lines and tubing and render the sterilant mixture unusable. Shelf life of the mixture is only a few months at best. It has now been unexpectedly found that when the surface of the vessels in contact with the sterilant mixture has been treated by exposure to carbon dioxide, the shelf life is extended. There is a substantial decrease in the production of ethylene oxide reaction products and the sterilant mixture has a substantially extended shelflife. In another embodiment of the invention, carbon dioxide gas is intimately admixed with the ethylene oxide and hydrohalocarbon. The carbon dioxide contained in this latter mixture achieves the iron oxide passivation in the vessel, avoids subsequent fouling, yet the overall mixture retains its favorable sterilizing, non-explosive and flame retardancy properties. This result using ethylene oxide, halocarbon and carbon dioxide is surprising since, the use of carbon dioxide in sterilant atmospheres had been shown to be a polymerization promoter. Tests have demonstrated polymer growth rates from ethylene oxides to be ten to twenty times faster in the presence of carbon dioxide than CFC-12. See Conviser, Stephen A., "Hospital Sterilization Using Ethylene Oxide--What's Next?", Journal of Healthcare Materiel Management, July 1989.
It would therefore be desirable to provide an improved ethylene oxide/hydrohalocarbon sterilant mixture which has a substantially reduced production of unwanted reaction products of ethylene oxide. Additionally, sterilization is carried out with the composition of this invention at acceptable pressures and contact times and without unacceptable mixture separation upon withdrawal from the storage cylinder.